The invention relates to an illumination system having a substrate and an active layer comprising an electroluminescent material, which active layer is present between a first, optically transparent electrode layer and a second electrode layer.
The invention also relates to a display device of the flat-panel type, comprising such an illumination system.
Electroluminescent illumination systems are used as illumination sources and in, for example, displays and indicator lamps (signaling). Such an illumination system is particularly suitable for use as background lighting for a display device having a liquid crystalline display panel (LCD device), for example, a polymer-dispersed liquid crystal display (PDLCD) device or a plasma-addressed liquid crystal (PALC) display device.
An illumination system of the type described in the opening paragraph is known from, for example, the article: "LEDs" by R. Friend, D. Bradley and A. Holmes in Physics World, November 1992, pp. 42-46. The illumination system described in this article comprises a semiconducting organic polymer film as an active layer which is formed on a first electrode layer which in turn is provided on a glass substrate. A second electrode layer is provided on top of the polymer film. One of the two electrode layers should be optically transparent to transmit light in a direction transverse to the plane of the illumination system. When a voltage is applied across the two electrode layers, the active layer emits light in accordance with an electroluminescence process. The active layer and the two electrode layers jointly constitute a planar light-emitting diode (LED).
The known illumination system has the drawback that the light usable for the formation of an image has a relatively low light output.